Video: Inside London's Deepest Tunnel

We're standing on London's deepest railway tracks, 75 metres beneath the surface. By the time you read this, the trains will be gone, replaced by a river of raw sewage that would easily swamp a double-decker bus. This is the Lee Tunnel, a brand new sewer running from Abbey Mills to Beckton.

Every year, 16 million tonnes of untreated sewage discharge straight into the River Lea. The tunnel is designed to capture that effluent before it enters the river. A gentle slope will then carry the feculent soup 4.3 miles east to Beckton for treatment.

The Lee Tunnel will eventually connect to the more famous Thames Tideway Tunnel, a new interceptor sewer, built on an even larger scale to greatly reduce sewage discharge into the Thames.

We took a tour of the Lee tunnel at the Beckton end, shortly before it was put into commission. Watch the video above, or see the captioned photos below to learn more about the project.

Looking down 75 metres into the overflow shaft. By comparison, Hampstead, the deepest tube station is 58.5 metres underground.Looking east towards the tunnel shaft. Now the sewer is operational, no one can stand here. The Lee Tunnel is designed to remain unvisited for a decade.Looking west towards Abbey Mills. The yellow pipe pumps fresh air along the tunnel.The rail lines move workers and materials along the 4.3 mile tunnel.Another view of the tunnel shaft. One of the giant concrete mixers used for lining the tunnel walls.Looking up to the surface from the floor of the tunnel.A train bearing concrete approaches. The growl is deafening.Londonist video editor Geoff Marshall (left) and Editor-at-Large Matt Brown (right) in full PPE.Beckton is itself full of interest. These old wheels date back to the original Victorian water treatment works.